Please select your interest from the table on the left - skirt design are in this section since you need to adjust your hull attachment points for the final skirt in a very early stage of construction. As well as select the appropriate skirt design before building anything on your craft. Please hold this in mind. 
If you work from plans of a designer please follow plan specs. for a successful project.

The basics of the lower hull:

As the lower hull of the craft we will include the craft floor, side panels, forward and aft panels till the top skirt attachment line. Most commercially build craft in polyester resin ( build out of a female mold ) will use this section to transfer to the top hull.
The lower hull :

• needs to have adequate size for the total weight of craft and payload.
• Must be strong enough to support craft  off cushion ( on landing pads). 
• Have enough freeboard to support craft in displacement mode on water.
• Must be watertight and as smooth as possible 

The above numbers shall help you to get a quick idea of the size you need for a given payload.

One of the biggest dangers operating  a Hovercraft are "plow in" ( having the nose dig in the water or sand during high end speed cruising) and "overturn" accidents. While overturn accidents can be mostly avoided by proper operation - plow in has to be held in mind while building the forward section ( bow section ) of the craft. Try to hold a boat like slope in the bow section of the craft which can extend forward of the skirt attachment line - this will not avoid plow in - but will make it a lot more comfortable than with a steep bow section.
The sides of the lower hull shall have a 15 - 30 degree angle between lower skirt attachment line and top skirt attachment line to minimize side "plow in". Or in case of a "plow in" to provide less damage to the skirt and a softer impact.  
This will provide us more or less with the shape of the lower hull -  at this point you need  the location for the lift air as well as lift unit ( engine, duct and prop or fan location) . Please hold in mind that the lift prop should not be at, or below the waterline if craft is floating under maximum payload.

Materials:

The lower hull can be build out of all boat building materials. From simple ply to very complicated composite panels. The lower hull is as well the section of the craft which might get the highest abuse during operation and especially landing in unknown areas. As long as your craft is on cushion there is no major harm against the hull - once your lift unit fails during operation - you can hope for a rigid floor or  a soft landing. Even if your craft has landing pads - if the center floor is nearly at the same level as the landing pads - you will only be able to set the craft smooth on a parking lot or water. All other surfaces will not be leveled enough to provide a smooth surface.
In case you use composite materials - try to use a core material which is easily available at your location. Continuously repairing a composite panel in the manner of filling just with resin and glass will over the years change the properties of this panel to a brittle and heavy panel. Not to mention here the side effects of having hard points in locations you do not need them.
We used common blue or pink foam  before changing over to Nidacore - they have the advantage of being easy available, reasonable enough to build quite thick panels for good flotation and once laminated on both sides with fiberglass in epoxy resin are chemical resistant. If a panel is only laminated on one side it is not a composite core material. Future damage is hard to repair and safety quite questionable.

To increase the strength of your floor you can encapsulate wood stringer in the center core. This will increase your weight minimal while providing maximum strength over the whole length of the lower hull as seen in the below picture.

3 layer of foam - center layer with internal stringer

Once you have your hull shape you have to select to provide inner strength to your craft by ribs ( as well known in  boat building as bulkheads)  or just wooden supports. 
On a cruising Hovercraft I would recommend that you give your hull absolute priority in aspect of strength - after that you can be greedy on the engine aspect. If the hull is not rigid enough to handle a certain abuse - you waste your time and can be stuck in the middle of nowhere with a collapsed hull. If you have length stringer in your hull or hull core panels try to laminate the ribs to them.
Good bonding will increase impact strength as well as the final layer of fiberglass cloth will seal moisture out. This construction is quite time intensive but worth the extra labor once you bump into something .... believe me there is a lot out there.