Hi
Purple,
Your going completly agains nature ref trying to block out Moles etc, (I'll write a mole and what its about later as a seperate page)
You'll not stop moles from digging through material, their main food are worms, Now worms can dig 1 mile under ground, they air the ground and through their defication (poo to you and me) make a thing called humus and this is what makes the food in the soil,it breaks down clay etc, you dont need to go to the fishing shop for these non garden insects to get worms in your raised bed, just mix soil and compost and the worms will come up via the un-touched earth under your raised bed, And the worms pull down leaves etc under the soil and as ive said eat these and make humus,
Ref weeds,
Just leave the grass we're it is on the bottom of your new raised bed, all plants (weeds are a form of plant) and all plants need Air, Light /water /food If you remove any of the four plant needs they'll die,
Covering up the raised bed ground with soil/compost removes the light provided its deep enough,
If you put turf upside down on the floor area of your raised bed you'll stop that grass from growing and you're making a start at a weed free
base.
ref MOLE hills caused by the moles pushing up the soil, this mole hill soil is ideal for potting on plants as the mole has done a fine job of airing this soil and the texture is just great for plants.
One of the best raised bed frame are the old railway sleepers (the wood the train tracks sit on) they are treated to stop insects etc, & are solid,