This whole foam set works as a system and can be used together to block larger openings or build walls across really large openings.
Foam companies tend to charge per cut so you can save some money if you follow these sizes.
Order a low density foam – it is much easier to use and actually tears less. You can make some useful smaller bits out of an old worn out block.
1 of 23 x 18 x 8 inches (large block – you will need to cut a slit from the top to just before ½ way down. Use a very sharp knife)
2 of 23 x 9 x 8 inches (same as above but cut in half lengthways )
2 of 18 x 9 x 8 inches (very useful size for stoves or smaller openings and as it’s the same length as the large block is wide.)
3 of 23 x 6 x 4 inches (keep one this size, cut one down to 20 and one down to 18 inches. Keep the off cuts to use as little fillers etc.)
3 of 23 x 6 x 2 inches (cut down to size as above)
For access hatches – order one piece 17 x 10 x 5 inches (cut this in to two bits measuring 10 x 10 x 5 and 10 x 7 x 5 respectively) I then make a diagonal slit from a corner to the centre of the square piece and a longitudinal cut from the short side to the centre of the rectangular piece.
Order small cylindrical bungs if you want but order as cubes and shape them yourself.
There are other useful sizes e.g. for the openings of semi-circular Victorian cast iron insert fireplaces
To kneel on you could use the highest density of foam (not reconstituted) about 21 x 6 x 4