Making my children's bike trailer more comfortable I'd like to know what I can do to increase the comfort and enjoyment of my two toddlers while riding in the bike trailer, typically for short to medium distances (3-4 miles most often, around a half hour or so).

I have an older model Schwinn two-kid bike trailer, the kind with no proper seat bottom (sling-style).  I have a 3 year old and a 1.5 year old child, and neither really cares for riding in the trailer - but particularly the younger one, who ends up squished.

I managed to fashion a seat out of a piece of wood just the right length to fit on the crossbars, which helped a lot on the squishing, but I'm curious what else I can do safely to increase the comfort of the children.  The trailer specifically says not to use a carseat, and I don't think I could fit two in anyway (or one plus fit the older guy).
A comfortable solution we used was to use a sort of hammock for our son. The model of trailer we have is a Croozer Kid, which offers a hammock that fits it frame. 

The hammock is attached to the top bar of the trailer, the front bar and the back bar. The third link to the back bar is needed to reduce bouncing. In between this triangle you can then span your fabric, with two soft bars on the side. In the hammock is a 5-point harness to strap the kids safe and tight. Below is a picture of the commercial model