I would definitely say do not try and modify your existing combo shell into a head shell - this is an easy one; three reasons:
1) It'd likely turn into a hack job as you'd have to cut thru the existing tolex.
2) Once it does turn into a hack job what are you going to do? At least if you try and build one from scratch and if it were to fail you could just pop your chassis back into the combo and your good to go!
3) Material to make one from scratch would be less than $20 and to replace your combo if it gets hacked would cost $100+
Now for the solution:
What I would do, and have done already with my .50 Cal+ (funny enough) is:
Buy a piece of 3/4" MDF from Home Depot, you can get a sheet 48" X 48" which will be more than enough, you can probably squeeze out 2 heads if you measure and make the right cuts.
You definitely need to visualize it before and while you are building it (as mentioned above by GTS). Make each cut carefully and draw it out first, even just a rough scribble it doesn't have to be to scale but you gotta see it in your mind first otherwise it'll go south rather quickly!
You can use your existing combo to get some of the measurements/most actually. MDF is strong and can be painted easily and takes quite well to spray paint, only problem is it chips easily. If you are looking to transport your head frequently then look into tolex or even that carpet like material you often find on car audio type sub woofers. You can get thin black colored that will be very easy to work with, tolex may be tricky if you aren't familiar or don't have the right adhesive.
Tools needed: Chop saw that can cut miters, carpenters glue (get the good stuff like Le Page), good 1 1/2" wood screws, drill with drill bits (pre-drill your holes!) and screw driver or driver bits for the drill. Sand paper, start with 60 grit and finish with 220 grit.
I'll tell you right now, getting the angle of the front of the head where the top piece meets the two side pieces is a bit of a ***** to get even-thoroughly think this thru when making your cuts.
As for the front piece where someone mentioned the grill or grill cloth, just cut a piece of the MDF to go in here and have it sit flush with the bottom piece and the two side pieces, looks awesome and it's simple.
This should at least set you in the right direction, it's necessarily hard, but you need to have a general idea of you to work with the tools and then just have patience and think it thru, don't rush it, I mean for $20 you can have two attempts at it.
By the way, you could also have Home Depot make some cuts for you, otherwise you will need a table saw to get the 48"X48" piece down to a size that fit your chop saw.
If when making your head, I'd cut my pieces 1/16" larger than I need, you don't want the opening of the head where the chassis fits thru to be too tight where you have to force it in.
Don't rush, and after you make each cut fit it to the piece it's attaching to to make sure you made the correct miter cuts!
Good luck!
If I find a picture of the one I made I'll post it, but I have a feeling I lost the photo's...I have since sold it...
