Remember your little one is moving from everything and everyone they know, and it’s as terrifying for them as it would be if you were put in a new home with new people. Attachment will take a long time, particularly given their attachment to birth mum and foster carer will both be disrupted.
Very much be baby led with this - for comparison, think how you’d feel being forced to be close to someone you literally just met. Your little one might cope with a sling, but might not, so have a back up plan, maybe a rear facing pram or buggy. You might need some activities and things you can do along side rather than with, if that makes sense - eg sensory stuff, hand painting, bath games etc rolling balls to them, building blocks alongside, watching the together etc. Try to match the closeness and intensity of activities with what the wee one is able to cope with. Ask the foster carers how they know when the baby is struggling or appears overwhelmed so you can attune to their needs.
I totally understand wanting to get stuck in to building attachment, the best way to build attachment is to be consistently responsive to your child’s needs, so that they come to trust that you will be there and are reliable. Your little one has some healing, grieving and processing to do before they’ll be able to trust that you’re not going to leave too - and at a very young age. It’s a bit of a dance when they’re so young and can only communicate using behaviour - you’ll become a bit of a detective.
Very best of luck, such exciting times for you - enjoy this time of anticipation and planning.