Bubble wrap, Reflectix, with some tape, or simply a double or triple folded over towel length-wise and a few bulldog clips have served me well over the years, not just with brewing. You also lose heat through the uninsulated lid, so lay something on top of that.
Since you're brewing small batches, I reckon you're using a stovetop? Can you leave the heat source on very low, or turn on intermittently, to keep your 150F for that 1/2 hour?
If you continue heating, depending on the kind of heat source, be careful with the type of insulation you'd use, and most likely refrain from using any to prevent it catching fire or melting, certainly with open flame or red hot coils.
Now I use an induction plate, so there's far less danger burning or melting anything as it is with other heat sources. For small 1-2 gallon batches an ~1800W induction plate should suffice.
I use a 3500W one for 5-6 gallon batches. During whirlpooling/recirculating I need to add 500-800W of heat to keep temps, without using insulation.
Isomerization (which turns hop oils into bittering compounds) tapers off fast (exponentially) at lower temps and is (virtually) non-existent at around 140F. Hop oil extraction also tapers off with lower temps, but will continue down to room temps or even lower (e.g., dry hopping), it just takes much, much longer. I don't know if it's bad to let your whirlpool/hopstand temps drop much below 140F, just keep the pot closed when not stirring and use good sanitation.