For tabletop work, I frequently use pieces of white or black foam core cut to the size needed, white to reflect light and black to absorb light. Some friends use flat white or black matt board the same way. Photographers that do framing tend to have matt board scraps available, I don't frame. I clamp a piece of wood to the bottom edge so they will stand easily on the table, or just prop them up with something if they need to be angled. Small A-clamps of several sizes help here too. I keep a fabric shopping bag full of small blocks of wood in various sizes to use for this and for building blocks to raise and lower my small props, much like how "Apple Boxes" are used in modeling shoots. Small pieces of black felt or black velvet and some chemistry stands come in handy too for hiding / covering these to keep them from being seen by the camera.
For larger needs, like portrait shoots, I made myself some large panels from insulation board, painted one side ceiling white and the other flat back. I can tape these together when a V-flat equivalent is needed, or place them each in a DIY stand with a slot to hold them upright. Each of these was made from 3 pieces of plywood. The bottom piece is about 12 X 12", and the two sides about 6 X 12". The gap between the upright pieces needs to allow your insulation board to slide easily in and out, so determine this by the thickness of your insulation board plus a little so it slides in and out easily. These stands are also the same white on one side and flat black on the other. My home studio ceiling is only 8', so the high dollar V-Flats are too big (too expensive for what they are too.). My DIY versions were cut to about 7'4" so they fit easily, even under my ceiling lighting support grid. I painted the walls and ceiling of my 19 X 26' DIY home studio shooting room is painted the same ceiling white color, so I can frequently bounce light off of them for larger shoots.
I have 5-Way reflectors in several sizes up to about 40", with several tripod reflector support stands, but use them when doing shoots outdoors more than in the studio. For outdoor shoots of people, I tend to travel light, using reflectors more than powered lights, as long as the Sun cooperates. It's much easier to carry several 5-ways than several 6-1200 w/s lights and batteries, so I usually take two or three Godox AD200 Pro II lights and reflectors. About all that I use the AD200's for is for fill when a 5-way doesn't quite do the job, or to get more light onpretty face.
I'm 85 now, so I no longer do many outdoor shoots, but when I do I try to travel as light as possible and keep the gear to a minimum.
Charley