I noticed a lot of interest in this so I decided to try the two-wood-block idea this morning and it worked great! Here's how to do it:
1) Two wood blocks (here in the US, I used 2" x 4" cross section wood, but anything close to that should work). Ideally the blocks will have hard, unrounded edges so that hot air does not escape under the blocks.
2) A heat gun that can go to a temperature of 950 degrees F (or around 500 degrees C if you use real units)
3) A piece of scrap cardboard
4) A test piece of acrylic (you'll want to try this out first to see what wood spacing, temperature setting, and exposure time work best for your conditions)
Place the piece of acrylic on top of the cardboard with the face you want to be on the outside of the bend facing up and position the two wood blocks on top of the acrylic so that a slit with a width of about 3mm goes over the area you want to bend. If you have additional 3mm thick acrylic, you can use it to separate the blocks and assure they are parallel. I realized I needed to take a photo of this afterward, so a different piece of scrap acrylic is shown in the image above
Turn on the heat gun and set it to around 500 degrees C. Actually, I never tried another setting- this just seemed "about right" but hotter or colder may work better. Hold the heat gun nozzle about 2 cm above the opening of the slit and move back and forward across the entire length (actually go a bit beyond entire length) of acrylic underneath the slit at a rate of no less then about 2cm/ sec. The silt will duct the hot air across the acrylic, but it will still be hottest directly below the nozzle, and you'll want the plastic to be heated evenly. Do this for about 3 minuets (again, I haven't really experimented with this so more time might work better).
Once you've sufficiently heated the acrylic, set the heat gun down, remove the blocks of wood, and pick up the acrylic. An early sign that it is ready to bend is that the acrylic will look a bit distorted where you heated it. Pick up the acrylic and slowly bend it away from the side you heated (the plastic will stretch to make an outer radius more easily on the hotter side). If heated properly, the plastic should be easily bendable by hand without much more effort needed then it would take to bend a coat hanger. Bend a few degrees past square since there will be some elasticity and it's nicer if the sides spring in toward the ROV structure anyway.
Smile at what a wonderful job you've done and get some ice cream to celebrate. Since this way of bending is not extreamly precise, the slots on the shell may not line up perfectly with the tabs on the structure. This can be remedied by using a shop file to extend the slots appropriately, and it is no cause to cancel your ice cream celebration!