I'd recommend the EHX Nano Q-Tron here for a no frills cheap plug and play mu-tron sounding envelope filter. Super easy to dial in using LP mode and the volume control can ensure no boost to your volume when you engage it. For a slightly higher budget, then I'd suggest the MXR BEF. That's a bandpass envelope filter but the ability to blend in your clean signal is a great feature, and, whilst it is up sweep only too like the Q-Tron, there's a bit more variety to the range of sounds you can get out of the MXR.