Gday Ralph!
Seems you are doing sufficient improvements to your howitzer...
As for the lightguards, I think you should invest in some flat brass wire. The effort involved would be no more than what you have already done for your current guards, and IMHO come out looking alright!

Made these for my M4A3. So long as you pick the right size, you will have a sufficient supply to last you a few tanks. An option I do is to heat the tip of a #11 blade (usually one that has had the tip snapped) and then press it into the tank at the pre-measured position where the guard is welded to the hull. This helps in a couple of ways. It gives the brass a 'hole' in which to be supported and glued, and also the rough heated lumps of plastic around the 'hole' can be either cut off later or used as part of the weld. If you have a hole running right through the hull, it makes it easier to get the exact measure for cutting.... As for the shape of the guard, well I use the tapered end of a paintbrush (after cutting approx. 2" length) Press the centrepoint of the length with the brush then bend around it to form a long U shape. USe some pliers to hold the top end at the right distance for the big 'curve' in the guard, then carefully bend the overhanging parts with your fingers to the required angle. After some trial and error, you will work out the position/angle from reference pics, and then cut the horizontal bars to length to attach to the curved guard in place. You will realise that the end will need to be cut at the same angle as the curved guard where they join... Same deal with the rear guards, just work from a reference pic and adjust for the angle of the hull....
HTH, I know I prattled on a bit, but don't be daunted!!
Cheers
Brad
P.S. As for the drain holes, they are for drainage of water/liquids trapped inside the splashguards around the turret and the fuel filler caps, the round air vents, think anywhere that water can be trapped. The holes to scale would be no more than the size of your thumbtip, and are at the outside base of the guards, at hull level.
Here area couple of examples
Credit M4 Sherman walk around

The circular splashguards on the rear deck have a hole in the outside centre at the base...