You can also use a sharp knife to slice little wedges out of the lower rim of plastic cups. That let's you cut any size or shape of hole that you want.
Or a soldering iron or wood-burning tool or a big nail heated in a gas flame.
>> in the past I used cell packs which were a pain because of irregular germination - I'd have to be digging out individual seedling to transplant into a pot
True, but the inconvenience is reduced if you use the tear-apart six-packs. Then you only need six cells to sprout close-together-enough. And sometimes you can double-up two six packs if each has one empty row, by nesting the full row from one pack into the top of the empty row from another pack.
When I use solid prop trays (plug trays), I cut them into 3-4 slices. For example, 10 rows (50 cells) become three slices with 3-4-3 rows.
Sixteen rows (128 cells) become 4 slices of 4 rows each - 32 cells per slice). Each slice gets just one variety, and usually germinate close enough together that I can pop out all the vigorous ones in one session, and ignore the rest. I expect late-sprouters to be less vigorous. But then, the most tricky things I start are Salvia and Delphinium, since I gave up on Penstemon. And Salvias and Delphs are not very tricky if you pre-soak them. And the seeds are not so precious that I can't waste a few.
I'm guessing that you start some difficult seeds that sprout over a range of weeks, and maybe "late-sprouting" does not mean "weak and sickly".
