Uthred
I have just stumbled on your thread here and as Oz directed you to one of my "tree-laden dios" I thought I might give you a couple of pointers. You are absolutely correct that purchasing large model trees from any supplier works out very expensive-that was the primary reason I had a go at making them myself and believe me you really have no need to be talented or possess any particular skill-they are very easy to make.
I have not currently got any underway to show you a stage by stage process but the attached unfinished tree (no base) may help you in deciding to have a go yourself-and if it all goes to rats it wont have cost you an arm or a leg.
The first couple of illustrations gives you some idea of how to start-but a good bit of advice First get and study some pictures of the real tree, oak, pine etc that you want to imitate. You will need three or four different gauges of binding wire-if you want a tree 12" high you will need 10 or 12 strands approx 36" in length-loop them in half and twist into a frame to form a skeleton with the heavier gauge wire forming the base. When you think the trunk is to the height you require start bending the wires to form your branches.
Building the trunk and branches-you can use all kinds of all purpose filler for this as long as it stays malleable long enough (must not harden too quickly) some modellers use Tetrion filler but I prefer ordinary duplex mix for car filler-a paste and a hardener that you mix together- keep building layer by layer until your trunk and branches appear to look right. The trick here is before the last layer fully sets take a hobby knife/carpenter's gouge or even a piece of wire and carve the bark efect/hollows/knots/dead stumps etc into the trunk and branches let it set. Paint in whatever color-dry brush the bark and give it a coat of varnish.
Foliage-now that you will have to purchase but it is not expensive-the illustration here is of "dead foliage" as I was going to construct a couple of Texas trees for the Alamo dio-but decided not to. You can get all kinds of foliage and I tend to use clump as my trees are more generic. Just stick it on with diluted PVA adhesive and spray with scenic cement.
All you need to do then is base it I use resin-warm it up and pour it into a flat mold and insert the tree then let it set. But you can stick it on flat styro-foam or hard-board even wood and cover the base with woodland scatter/hairy grass or a variety of coverages. Illustrations show a "generic Joshua type hybrid" a Tall Pine and a "generic squat oak"
If you decide to have a go and you want some more detailed guidance just let me know
Reb