You can do it with super-Gaussian function:
using GLMakie
fig = Figure()
range = LinRange(-1, 1, 5) # LinRange{Float64, Int64}
p = 4 # supergauss order
a = 1 # radius
cube = [@. exp(-(x/a)^p) * exp(-(y/a)^p) + exp(-(z/a)^p) for x = range, y = range, z = range ] # Array{Float64, 3}
ax = Axis3(fig[1,1], aspect = :data, azimuth = 1.17 * pi, viewmode = :fit, title = "Cube" )
volume!( cube, algorithm = :iso, isorange = 0.05, isovalue = 1.7 )
fig
Higher order
p will give more sharp edges.