diorama supplies......

There are many different methods to do this and several books on the subject, starting with Shepard Paine's; How to Build Dioramas. Osprey's; Terrain Modeling, or WWII Dioramas are other good choices. If your diorama is relatively flat, you can build your ground work right on your base. If more elevated terrain is needed, you need to build up the substructure first. One of the easiest and cheapest methods is by using styrofoam blocks or sheets, cut to fit the rough contours needed and then covered with a plaster/paper mache mix such as Sculptamold, Celluclay or other similar product. This is then sculpted with a spatula or other tools into your finished contours. Sand, rocks, dirt, dried peat moss, static grass and other scenic materials can added after the Celluclay dries. Paint as desired. I also use plaster bandage material over a foam core for the same result. You can even use the spray foam insulation in a can if you need to build the core of a mountain side or something else really large, but this takes some practice, technique and a bit of skill to avoid creating a large, gooey mess!

If your useing celluclay, you can also colour it as you mix it. I use cheap acrylic paint, the stuff you find in childrens sections or art stores. There is a huge range of materials you can use to cover it. I ue a wid range of scatter materials, mainly from woodland scenice, as well as grass tufts, rubberised horsehair.

There is also a wide selection of paper plants that i have been useing recently and they can really add to a base.

Here are a few of my recent ones and you can see a large selection of materials being used.

Yes. I did purchase a good book on basic diorama work written by laslo adoba, which is good for starting out. Would it be somewhat historically accurate to place a Hochkiss H 39 dug in by a trench? 

I am assuming you want to use the H39 as a captured vehicle under German use? During it's rather brief career with it's original owners, I highly doubt it would have been used in such a fashion, due to the rapid advance of the German Army and fluid nature of the Battle of France.

The Germans used large numbers of captured French tanks, mostly in anti-partisan and other second-line duties. Many were stripped of their turrets for use in Atlantic Wall pillboxes and their chassis used for towing vehicles, ammunition carriers, SP guns and SP AT guns. Units using unmodified or slightly modified French tanks included Panzer Abteilung 206, Panzer–Ersatz und Ausb. Abt. 100 and Beute-Sturmgeschütz-Abteilung, among others. By December 1944, only 60 Hotchkiss H39s were still active. While I have a fairly extensive library and photo collection, I don't have, nor have ever seen, a H39 in a dug-in position, but I stand by "Never say Never". I don't have much info on the previously mentioned Beute and Ersatz units and only a few photos, none of which are helpful for your situation.

A overview of captured French vehicles under German use;

http://www.axishistory.com/books/138-equipment/equipment/4260-captured-a-converted-french-vehicles-in-german-service

If you want to do this scene with a KOed T-34, I've got several nice photos of them incorporated into trench lines.