Anonymous Monk, you could

1. do your bell curve trick to produce a standard normal ∈ (−∞, +∞) centred on 0
2. map the negative half (−∞, 0] thru exp onto 0,1 and thence thru ƒ($num) = x over {1 + y • (1 − $num)} onto [x/y, x].
3. map the positive half (0, +∞) thru g($rand) = 1−exp(−$rand) onto (0,1) and thence thru h($num) = x • (1 + $num • y) onto (x, xy).

. That will be centred on x, mound-shaped, and put half the probability mass on either side of x — stretched according to your whims.