Good question. On molds where there's just a figure I tend to use backing boards (rough side in for increased air flow) and the orange/green spring clamps (the type found at Lowe's or Home Depot..I have like 10). For tricky thin pieces or 'heads' where the face doesn't form, I bought the really thick rubber bands at Office Depot and while still using backing boards to apply consistent pressure, they do allow expansion and increased detail. Atmospheric pressure does make a difference. High pressure days are superb for casting whereas low pressure days can yield variable results. Humidity also plays a part. I'm thinking of having anther go at the '75'. Let me have a bit to play with it and I'll tell you my final results. It's not a bad piece but it does seem to be ...'abbreviated.' It's almost on a smaller scale (52mm perhaps) and the trail seems short, as does the barrel. My guess is that when Rich first had it sculpted, he didn't consider doing the long mold like the MM PH. that would be ideal. The shield, barrel, and wheel should be in a long mold and the trail should be in another. If I was going to design it, I would have it be 6 pieces (trail, shield, wheel, barrel/breech, rt cross bar and seat, left cross bar and seat.) Probably would be a 30 dollar mold but worth it.
For variation, you can use the Fr. officer holding hand up, Fr. officer with map, and loader. If you want a US set, you can use the Brit. Mule handler for firing, the US mule handler for an 'assistant' (IE: standing around) and the US shell handler and the Officer firing. This gives you an accurate crew.
