well with the poly you will probably have to glue sheets together to get the shape that you need ,you can use aquarium silicone to do this i would integrate the background into the land piece and make it as one piece that will make it more natural in the end. to cut poly use a sharp stanley knife carefully with the blade most of the way out to shape it there are many tools you can use a lot of different kitchen utensils work well . you can also use rasps and surforms and if you are careful a heat gun. i have a heat gun you can borrow and i have somewhere a small rasp i found really helpful as one end is curved (it may take me a while to find it in my hgarage though i also have a lot of dark brown acylic house paint and some lighter stuff for dry brushing which i have used on my backgrounds so i know that they are safe to use . you are welcome to use this too. i would suggest hollowing out the poly into a shape to hold soil where you plan to plant your plants , the bigger the better as more soil will mean healthier plants . once carved use paint straight on the poly and when still wet sprinkle sand and gravel in differet areas to add texture this can be painted over again when dry to help hold it in place and use the other colours dry brushed on to add interest and high lights. is the filter you plan on using an internal one? if so you could cut out a gap in the corner of the tank to fit it in and build the side of the poly around it so that it can still be removed but has a removable poly lid or similar