My tent goes in a pannier for this reason, with the top of the rack space used for water, drying laundry and an
Alpkit Airlok Extra that comes out to provide extra capacity when needed for shopping etc. The latter has attachment points for additional adjustable webbing straps, and I use these to secure it to the rack. I dare say the same approach could be used for a tent.
Toe straps are the canonical cyclist's solution to short ratchet straps, but they may not be long enough to secure your tent.
I have some light duty cam-buckle tie-down straps (a couple of metres long) that are generally useful for securing things on the trailer. They came from the car stuff section of Wilko. I generally carry one when touring because it's so useful for securing bikes on trains, as well as being handy for random acts of bodgery (I've used it in combination with a convenient tree as an improvised recumbent-friendly workstand before now), but it might be a bit unwieldy for this purpose, compared to a pair of lighter duty straps.