Only some small demolition jobs today then Vic?!!
That plastic might work, but it might be a bit thin, which can cause uneven and unmanageable edges, as well as possible thin spots, bubbles or holes. This is because the clear plastic looses dimensional stability when heated, and of course goes thin as it stretches over the former. I'd suggest you try it on a rough balsa mould first, unless you can get some clear sheet from a model shop. Most good shops sell it, in 8 x 10 inch or slightly larger, in various thicknesses. For this job, anything between 0.5mm and 1 mm thick (approximately) will be fine. Once moulded, the 0.5mm will be down to around 0.25 to 0.025 mm thick, and the 1mm around 0.5 or maybe a bit less.
As a rough guide, cut a piece which is approximately 4 to 6 times the size of the balsa male mould in length and width in plan view, and attach this firmly to the female mould, with the moulding aperture centralised.
If you have a kitchen stove with gas hobs, these are ideal to heat the sheet, as there is more control, and the effects are immediately visible, compared to electric or infra red hobs, or an electric grill.
The clear sheet can be fixed to the female mould sheet (this something like 1/4 inch balsa sheet, or equivalent hard, stiff card.) with ordinary masking tape or 'Sellotape', but ensure this is well stuck down, with the ends wrapped around the clear sheet and the mould, and on the opposite side to the heat source. (this to prevent the heat from lifting the tape ends.)
Hold this combination over the flame/heat source, with the clear sheet uppermost, at a height of no less than around five or six inches (wear gloves or use tongs!), and gently move it around in a circular motion to prevent fogging or melting. When the clear sheet starts to go 'floppy' and wants to curl up and pull away the tape, immediately plunge the male mould through the sheet in one smooth, swift, but gentle motion, until the base of the male mould is level with the top surface of the female mould, or at least inside the thickness of the female mould.
Allow to cool for a few seconds, then remove the male mould. Allow a few more seconds to cool, then remove the now baked-on tape, and take the moulded sheet off the female mould, carefully lifting the moulded shape through the aperture.
The male mould can be placed into the moulded shape to act as a cutting guide, and the part removed by cutting with a new blade in the scalpel. The best way is to continuously, but gently and carefully score the cut lines, to avoid splits or slipping of the blade.
The part can be fixed to the model with poly cement or PVA.
Although it can happen, don't expect a perfect result first time, even with experience of doing this, as the control parameters are ... er... there aren't any !
Forgot to add - it's best to make the male mould deeper than the required shape of the part, to allow room for pushing through the female mould, and for ease of cutting. With a part for say, a Hurricane canopy, the male mould would have the required sloping sides, but from the base (of the canopy shape) to the bottom of the male mould would be vertical all round.