Can anyone suggest a company that can make a reasonably cheap display case to go over a diorama ?

Cheap+Custom+Display case= does not add up.

Your best bet is to make one yourself out of sheet acrylic and some square rod for the corners. Just remember the thicker the sheet the more it costs building display cases is alot easier then it sounds i have made about 20 for myself in the past two years.

would you have a kind of template or guide to building them you could give me

Have a look in discount stores. I've picked up small acrylic fish tanks for $5 - $10. Even if you don't buy them they'll give you an idea as to construction. Making them should be easy. Just measure your four sides plus top and you may be able to get you friendly acrylic sheet supplier to cut them for you.

Cheers

Best way is if you are wood worker build a Display case.

Or a Curio cabinet.

http://www.woodcraft.com/family.aspx?FamilyID=6711&wcs=6831&pcs=fam

http://www.how-to-plans.com/craftsman_curio_cabinet_plans.html

I have no set template each case was built to house items differently IE: multi shelfs, hinged doors, enclosed and sealed so they were each made to be custom to that need. This is why taking a bit of time and building some cases is really the best way to go because you will ensure you get just what you need and want. All you have to do it plan it out and you won't be bound by some premade item or having to pay some one else for their time, materials.