[dice]

You could check local landscape materials supply companies for lava
rock, pumice, or "decomposed granite" (granite dust and gravel from
quarries) to mix into your peat. (You would probably need to add a
handful of dolomite lime per container for a source of calcium and
magnesium and to raise the pH of the peat from the around pH 5 range
up to between pH 6 and 7.)

An alternative would be to mix bark fines into it (pine or fir bark, shredded
as fine as you can find). Bark is somewhat hydrophylic (repels water) and
adds air space without adding much weight.